I'm very interested in this as well as I have a spare 15 high for LPS and Zoas that I would like to plumb into my existing 40 breeder system. Some watchouts so far from my own planning are:

Make sure the return pump can pump up the extra water coming from the second tank
Check to make sure you have plenty of flood space in your sump when the power goes out. From my thoughts, you will have twice as much water flooding the sump if you keep your overflows/ returns at the same height as the original tank; meaning you will need almost twice as much available flood room in your sump for the extra water.

I am curious on what others feel are watchouts